Electric superchargers, also referred to as superchargers, boost assist devices, or E-boosters, may be adapted to turbocharged engine systems to reduce turbo lag and increase power output of the engine during certain engine or vehicle operating conditions. In particular, during low engine speeds when a turbocharger may experience difficulty in providing a desired compression, an electric supercharger may assist in boosting intake air. While the turbocharger includes a compressor mechanically driven by an exhaust turbine, the electric supercharger includes a compressor that is electrically driven by a motor. The electric supercharger may be staged in series or in parallel with the turbocharger in what may be referred to as a compound boosting configuration where the electric supercharger may be utilized to increase the transient performance of the turbocharger. By coupling the electric supercharger to turbocharged diesel or gasoline engines, an engine may be downsized without compromising peak power and torque performance.
In V6 or V8 engines where the engine may be configured with two banks of cylinders, twin turbochargers may be used. In such systems, each turbocharger may be coupled to an exhaust manifold of one of the cylinder banks, relying on exhaust gas generated during combustion to drive a rotation of the turbocharger turbines. An example of a dual turbocharged engine with two parallel turbochargers is shown by Banker et al. in U.S. 2011\0265771. Therein, an engine system is shown with twin parallel turbochargers for improving the boost supplied to an engine with dual cylinder banks. An air intake passage upstream of the twin turbochargers splits into two parallel intake passages that include the two turbocharger compressors. Thus the turbochargers provide boost to each bank of cylinders.
The engine system described in U.S. 2011\0265771 also includes low pressure exhaust gas recirculation (LP-EGR) loops coupling the exhaust manifolds of each cylinder bank to the intake passages upstream of the compressor as well as high pressure EGR (HP-EGR) pathways coupling the exhaust manifolds of each cylinder bank to the intake passages downstream of the turbocharger compressors. EGR allows a reduction in concentration of undesirable combustion byproducts such as NOx and particulates in the exhaust gas. Exhaust gas flowing from the cylinders through the exhaust manifold may be diverted from the exhaust turbine to the EGR loops or directed through a wastegate to the atmosphere after passing through an after treatment device, such as a catalyst. Gas flow may be selectively divided between a combination of the paths described above, depending on vehicle operating conditions.
The exhaust-driven turbochargers may be configured as variable geometry turbochargers (VGTs) where nozzles of the turbines may have vanes that vary the cross-sectional area of the turbine nozzle, thereby adjusting the rotational speed of the turbine wheel and the amount of boost delivered to the engine. VGTs may also be used to control EGR by increasing or decreasing the exhaust manifold pressure as a result of narrowing or widening the turbine nozzle flow area. The efficiency of the VGT may therefore affect EGR flow. For example, during engine transients, the turbine nozzles may be adjusted to decrease the flow area of the nozzles, increasing the EGR flow rate which may lead to a decrease in NOx in the exhaust emissions.
Electric superchargers, in addition to reducing turbo lag, may also assist during engine transients by supplementing the boost provided by the exhaust turbochargers when the exhaust turbochargers are unable to meet the boost demand. However, the typical positioning of the electrical superchargers upstream of the exhaust turbochargers may lead to additional complications. As an example, electrical superchargers often operate within a limited flow range of the turbocharger compressor, such as the surge and choke regions.
Compressor surge may occur, for example, when an operator tips-out of an accelerator pedal, resulting in decreased air flow and leading to reduced forward flow through the compressor at high pressure ratio (PR). In another example, surge may be caused in part by high levels of cooled EGR which increase compressor pressure while decreasing mass flow through the compressor. Compressor choke may be encountered at high flows, when an increase in compressor speed gives a diminishing increase in the rate of flow. When the flow at any point in the compressor reaches the choke condition, no further flow rate increase is possible. This condition represents the maximum compressor volumetric flow rate as a function of the pressure ratio. As one example, choke may occur when an operator tips-in from a part load or idle conditions to a high load condition, such as when going uphill with a load.
In order to widen the range of mass flow through which stable operation of the turbocharger compressors may occur, a diameter of the compressor wheels of the superchargers may be increased. However, increasing the compressor wheel diameters may result in higher pressure at the supercharger compressor outlet and thus higher pressure at the turbocharger compressor inlets, lowering efficiency of the turbocharger compressors.
Various approaches have been developed to address the turbocharger inefficiency resulting from coupling with a supercharger. One example approach to address this issue is shown by Rutschmann et al. in U.S. 2016\0258348. Therein, an internal combustion engine boosted by an exhaust turbocharger is disclosed with an electric supercharger arranged downstream of the turbocharger. The supercharger is positioned in a bypass to the main intake passage and air flow through the main intake passage may be diverted by closing a check valve in the main intake passage. Air that is not sufficiently boosted by the turbocharger compressor may then be additionally compressed by the supercharger when the check valve is closed. When boost pressure from the turbocharger compressor matches the requested amount of boost or torque, the check valve is opened, an electric motor of the supercharger is switched off, and air is delivered directly from the turbocharger compressor to a throttle valve. Thus, the supercharger assists the turbocharger in meeting boost demands while minimizing energy consumption. Furthermore, the boost assistance provided by the electrical supercharger may be adapted to systems with more than one turbocharger upstream of the supercharger.
However, the inventors herein have recognized potential issues with such systems. As one example, in engines with dual cylinder banks, it may be desirable to couple each of the twin turbochargers with an electrical supercharger directing boosted air to one of the cylinder banks to avoid cumbersome merging and splitting of intake passages. The incorporation of two superchargers in the vehicle front end, however, incurs higher costs as well as greater space requirements to accommodate the devices. Synchronization of the independently operated superchargers may impose complexity of control as well as undesirable air flow responses.
Further, the component life of the superchargers may be limited by frictional forces imposed on the supercharger arising from its configuration. Conventional electrical superchargers comprise an electric motor coupled to a compressor wheel via a shaft. The asymmetric arrangement of the supercharger may result in unbalanced frictional, or thrust, force in an axial direction, e.g. in a direction from the compressor wheel towards the electric motor. A thrust bearing is used to compensate for the thrust force and the effective lifetime of the supercharger may be based on degradation of the thrust bearing to a point where the thrust bearing may no longer mitigate the thrust force.
In one example, the issues described above may be addressed by a method for a supercharger, comprising an electric motor including a first output shaft and a second output shaft positioned on opposing sides of the electric motor and sharing a common rotational axis, a first compressor rotationally coupled to the first shaft, and a second compressor rotationally coupled to the second shaft. In this way, a single electric motor may simultaneously drive the rotation of the two compressor wheels of the double-ended electrical supercharger (or supercharger) to provide boost assistance to twin exhaust turbochargers.
As one example, an engine system includes a double-ended supercharger arranged downstream of twin exhaust turbochargers, at a location downstream of where air passages from the turbocharger compressor outlets merge into a single channel. The supercharger is configured as a doubled-ended supercharger wherein a first and a second output shaft extend from opposite sides of a single electric motor. A first compressor wheel is coupled to the first output shaft and a second compressor wheel is coupled to the second output shaft in a symmetric configuration to balance thrust forces along the axial direction. The supercharger may be positioned in a bypass passage coupled to the main intake passage downstream of the twin turbochargers. A bypass valve may be disposed in the intake passage that diverts air flow to the bypass passage when commanded to close based on engine operating conditions. For example, a controller may command the bypass valve to close responsive to low engine loads where boost provided by the turbocharger compressors is sufficient to meet the boost demand. By closing the bypass valve, intake air is flowed directly from the turbocharger compressor to the engine cylinders via a main intake air passage.
In this way, by configuring a supercharger with two compressors driven by a single electric motor, the two compressors of the double-ended supercharger are automatically synchronized, reducing the complexity of control in comparison to configurations with two distinct electric superchargers coupled to distinct turbochargers of a twin-turbo system. By incorporating of the two compressors onto a single supercharger, costs are reduced and packaging constraints of a boosted engine system may be better met. By relying on a single set of electronics and a common bypass valve to control air flow through the supercharger, control complexity of the boosted engine system is reduced without compromising boost control. By relying on a double-ended configuration, the supercharger may operate in a high efficiency zone with a wider flow range and improved vehicle transient response without degrading exhaust emissions. Furthermore, the component life of the supercharger may be increased by balancing axial thrust forces with the symmetric configuration. Overall, the boost assistance provided by the double-ended supercharger as well as its mechanical efficiency may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.